It is desirable to maintain clear vision through windshields during the presence of rain or water spray. In fast moving vehicles, such as aircraft, mechanical wipers are not totally effective in removing rain or water spray which impact the windshields in large volumes and at rapid speeds.
There are numerous conventional rain repellent compositions for airplane windshields. These compositions are stored in pressurized canisters located in or near the airplane cockpit. The repellent composition is carried by pressurized tubes to spray nozzles located adjacent to and below the exterior surfaces of the airplane windshields where the repellent is applied to the windshield.
Some of the conventional rain repellent compositions include a highly volatile solvent, such as trifluorotrichloroethane (also known as Freon, a trademark of Du Pont de Nemours, E.I. & Co.) and a hydrophobic polymer. Freon is often used because it has excellent solvent qualities, is nonflammable, has low toxicity, and it rapidly evaporates when applied to a windshield.
When the rain repellent composition is applied to a windshield, the windstream causes the solvent/polymer to be dispersed across the windshield. It is desirable that the hydrophobic polymer be spread across the windshield as a thin film which is hydrophobic in nature. In this manner the rain droplets, instead of adhering to the windshield, bead up, and are blown off the windshield by the airstream. Freon, once applied to the windshield, flashes off quickly leaving only the polymer film. However, the high volatility of the Freon solvent can also be a disadvantage. For example, in conditions of no rain or very light rain, the Freon may flash off before the polymer has had an opportunity to be dispersed across the windshield. This not only leaves a portion of the windshield without rain repellent, but the portion of the windshield having the rain repellent may not be optically clear due to the large amount of rain repellent present there. This is sometimes referred to as the "dry smear" problem.
There have been a number of conventional compositions and methods for rendering optical surfaces, such as aircraft windshields, repellent to water. For example, in U.S. Pat. No. 3,433,750 by Fain et al, the contents of which are incorporated by reference herein, there is disclosed a rain repellent compound including a cationic, surface-active quaternary ammonium component and a silico-titanium copolymer component. Furthermore, in U.S. Pat. No. 3,766,123 by Burnie et al there is disclosed a rain repellent compound including a copolymer of the silicotitanium type, silicosilicon type or silicozirconium type, a spreading component, and a solvent component. In addition, U.S. Pat. No. 2,512,058, the contents of which are incorporated herein by reference, discloses a method of forming silico-titanium copolymers.
In addition to the problems caused by "dry smear", many of these conventional rain repellents are odorless and are difficult to detect in the event of a leak in the repellent container or distribution line which carries the repellent to the spray applicator.